1. Field of the Invention
The present invention relates to an electrically conductive resin composition containing metallic powder, as a main electrically conductive material, capable of exhibiting good electrical conductivity and stable electrical conductivity for a long term even at a low content of the metallic powder.
2. Description of the Related Art
Recently, remarkable developments have been made in various electronic devices or equipment such as computers, VTR, audio devices, home electric appliances, and word processors, but these electronic devices have disadvantages in that erroneous actuation can occur due to electromagnetic waves from the outside, since the electric current used therein is very small because of the use of highly integrated electronic circuits and elements.
Various plastic molded articles are widely used as housings of the above-mentioned electronic appliances because of the need for light weight articles, cost-reduction, and mass production. Plastic molded articles do not provide a shielding effect against electromagnetic waves and, therefore, the electromagnetic wave is transmitted through the plastic molded articles. Thus, the above-mentioned disadvantages cannot be solved by these plastic molded articles.
Various attempts have been made to provide plastic molded articles with shielding effects against electromagnetic waves (i.e., electromagnetic wave shielding properties). For example, electrically conductive resin compositions containing metallic powders uniformly incorporated thereinto have been proposed. The amounts of the metallic powders contained in the electrically conductive resin compositions should be reduced in view of the specific gravity, cost, and mold processing properties of the electrically conductive resin compositions, but, when metallic powders such as copper fine powders are used, the desired sufficient electrical conductivity and electromagnetic wave shielding properties cannot be obtained unless 80% by weight, based on the total amount of the resin, of the copper powders are contained therein. When the resin is polypropylene having a specific gravity of 0.91 g/cm.sup.3, the specific gravity of the resultant resin composition is heavy, for example, 3.2 g/cm.sup.3, and the mold processing becomes difficult so that the use of practical plastic molding processes such as injection molding processes becomes unpractical.
The use of powders of inexpensive metals other than the noble metals, such as nickel, copper, iron, and aluminum, as an electrically conductive material, is preferable in view of the low cost thereof. However, since these metallic powders are susceptible to oxidization in the resin compositions, an electrically non-conductive oxide layer will form on the surface thereof and, therefore, the electrical conductivity and the electromagnetic wave shielding properties are reduced with the lapse of time. Thus, practically satisfactory products have not been obtained as yet.